The invention relates to a hydraulic control arrangement for a mobile machine comprising at least two hydraulic loads, one of them having a lifting function.
In U.S. Pat. No. 6,293,099 B1 a control arrangement is described which is provided for controlling the loads of a forklift. A forklift usually has a boom along which a fork is movable to lift or lower a load. The boom can moreover be tilted and laterally displaced, each of such functions being actuated via hydraulic cylinders. For controlling the pressure fluid volume flow to the respective hydraulic cylinder, in the known solution proportional valves are provided through which the pressure fluid volume flow to the respective hydraulic cylinder can be adjusted. Pressure fluid is usually supplied via an LS variable displacement pump or a constant displacement pump including a bypass pressure regulator. The variable displacement pump or the bypass pressure regulator can be controlled, in response to the maximum load pressure of all loads, so that the pump pressure is above the maximum load pressure by a predetermined Δp.
In such control arrangements a problem may arise when, for example, the empty unloaded fork is to be lifted and simultaneously the boom tilt is to be varied. In this case, the load pressure at the tilting cylinder is higher than the load pressure of the lifting cylinder so that the latter is preferably supplied with pressure fluid and the fork is lifted comparatively quickly, while the tilting motion is performed very slowly or is even stopped. This problem arises practically with all conventional forklifts.
It could be attempted to arrange in the pressure fluid flow path to the tilting cylinder an LS valve including metering orifice and pressure regulator to which the pressure upstream and downstream of the metering orifice is applied. In this case, lifting of the fork and tilting of the boom could be simultaneously performed, unless the maximum possible pump delivery is exceeded. In the case of undersupply, i.e. in case that the pump cannot deliver sufficient pressure fluid, the load having a higher load pressure slows down because the pump pressure prevailing upstream of the metering orifice thereof is dropped and thus the pressure difference above said metering orifice is reduced—the same problem as with the control arrangement including proportional valves is arising.